Electronic ignition system with encapsulated reed device and conducting sleeve thereabout forming a flux filter



Nov. 8, 1966 N. T. NEAPOLITAKIS 3,284,740

ELECTRONIC IGNITION SYSTEM WITH ENCAPSULATED REED DEVICE AND CONDUCTINGSLEEVE THEREABOUT FORMING A FLUX FILTER Filed July 15, 1965 FIG.1

IO! 89 8? K FIG-3 w 3 INVENTOR NICHOLAS T. NEAPOLlTAK\S Wm M v ATTYS.

United States Patent ELECTRONIC IGNITION SYSTEM WITH ENCAPSU- LATED REEDDEVICE AND CONDUCTING SLEEVE THEREABOUT FORMING A FLUX FIL- TER NicholasT. Neapolitakis, Chicago, Ill., assignor to Motorola, Inc., FranklinPark, Ill., a corporation of Illinois Filed July 15, 1965, Ser. No.472,138 4 Claims. (Cl. 335-453) This invention relates to magneticallyactuated reed switches, and more particularly to improvements in systemsutilize a magnetically actuated reed switch.

'Magnetica'lly actuated reed switches have numerous applications inwhich they are opened and closed at a certain frequency. For example,ignition systems for internal combustion engines may utilize amagnetically actuated reed switch to synchronize operation of the systemwith the operation of the internal combustion engine in order to producethe firing pulses at the proper time.

Such a system is described in the co-pending application of Robert C.McLaughlin, Serial No. 343,652 (assigned to the assignee of the presentinvention). This involves utilizing a rotating magnet proximate the reedswitch to produce flux variations at the desired frequency.

Under certain conditions, reed switches may tend to bounce in thepresence of spurious flux variations at the natural resonance frequencyof the reed switch. Spurious flux variations at the resonance frequencyof the reed switch may result from vibrations at such frequency or atfrequencies to which the resonance frequency is harmonically related.These vibrations may be produced due to play or variation in dimensionsin the mechanical structure of the switch actuating mechanism, tovariations in power supply current which produce spurious fields, or byother causes.

Accordingly, it is an object of this invention to improve the operationof systems utilizing magnetically actuated reed switches.

Another object of the invention is to reduce the tendency for reedbouncing in a system which utilizies a magnetically actuated reedswitch.

A feature of the invention is the provision of a magnetic shieldsurrounding the reed switch of a system, which \magnetic shield has aninductance sufliciently low to have a negligible affect in fluxvariations below a given frequency and which operates to filter out fluxvariations at frequencies of the order of the natural resonancefrequency of the reeds.

Another feature of the invention is the provision of a copper sleevesurrounding an encapsulated reed switch, which sleeve filters outspurious flux variations which can cause vibration of the reed due tothe natural resonance thereof.

In the drawings:

FIG. 1 is a schematic diagram of an ignition system incorporating theinvent-ion, and showing the reed switch and shielding structure in fullsection;

FIG. 2 is an end view of the reed switch and shielding structure of FIG.1 taken along the line 22; and

FIG. 3 is a full section view of a distributor in which the invention isincorporated.

In accordance with the invention, an electrical system includes amagnetically actuated reed switch operable to control a function of thesystem. Magnetic means are provided adjacent the reed switch and produceflux variations to open and close the switch. The reed switch has anatural resonance frequency which is substantially higher than theoperating frequency. A magnetic shield surrounds the reed switch. Themagnetic shield has an inductance suificiently low to have a negligibleaffect on flux variations at the operating frequency and operates tofilter out flux variations at frequencies of the order of the naturalresonance frequency of the reed switch.

Referring now more particularly to the drawing, FIG. 1 shows theinvention incorporated in an ignition system for an internal combustionengine 11. Engine 11 has a plurality of spark plugs 12 for igniting thefuel mixture in the cylinders of the engine. High voltage firing pulsesare supplied to spark plugs 12 by a distributor 13 which has a pluralityof fixed contacts 14 connected to a respective ones of spark pulgs 12,and a moving contact 15 connected across the secondary winding of a highvoltage ignition coil 16. As shown in FIG. 1, ignition coil 16 is of theauto transformer type, howeventhe double winding type of transformermight also be used. When current flowing through the primary of ignitioncoil 16 is suddenly interrupted, a high voltage spike will be introducedin the second thereof. It is this spike that is applied to movingcontact 15 of distributor 13 and from there to one of the spark plugs 12of internal combustion engine 11.

The primary ignition coil 16 is connected to the source of current,namely storage battery 17, by a transistor 18. Battery 17 may be chargedby an alternator or generator 21, the output of which may be regulatedby regulator 23 in a well known manner. Also connected in series withthe primary of coil 16 and transistor 18 are the ignition switch 25 andballast resistor 27. A protective zener diode 29 is connected across theemitter and collector portions of transistor 18. The base portion oftransistor 18 is connected through resistor 31 to the emitter portion ofthe control transistor 32. Transistor 32, when conductive, willtherefore cause resistor 31 to forward bias transistor 18 intoconduction. When transistor 32 is cut oil, the resistive choke 33 whichis connected across the emitter and base portions of transistor 18 willproduce a transient pulse to reverse bias transistor 18 cutting it off.This interrupts current through coil 16 and produces a firing pulse. I

Conduction of transistor 32 is controlled by a resistor 34 connectingthe base portion of transistor 32 to ignition switch 25, and by amagnetically actuable reed switch 45. Reed switch 45 comprises a pair ofreeds 47 and 48 which are normally open. Reed switch 45 is glassencapsulated and placed adjacent a rotating magnet 50 which, during thecourse of'its rotation, will align pole pieces of oppositepolarity'adjacent respective ones of reeds 47 and 48. This causes thereeds to attract one another and therefore to close. The alternate polesmarked N, S, N, etc. around the periphery of the annular magnet areuniformly spaced to operate the reed switch at regular angular positionsof the magnet. As the magnet 50 oontinues rotation, the alternate polesproduce fields which are in opposite direction to open the reeds at thehalf way point and then close them again once the field is completelyreversed. It should be noted that the poles at the half way point mayactually force the reed apart in the event that they tend to sticktogether due to magnetic retentivity or moisture.

With reed switch 45 closed, the base port-ion of tran sistor '32 will begrounded and hence transistor 32 will be driven into conduction to turnon transistor 18. When reed switch 45 opens, the base portion oftransistor 32, which is connected to battery 17 through resistor 34,will rise to battery potential, actually reverse biasing the transistor32 to insure rapid cut off.

Magnetically actuabl'e reed switches of the general nature of switch 45are commercially available from numerous sources. One such source isHamlin, Inc. of Lake Mills, Wisconsin, under the designation MRG-l.

The glass encapsulating tube may be approximately one eighth inch indiameter and less than one inch long. The reed may be a nickel ironalloy and rhodium plated at the contacting portions. As an alternative,the reeds may be gold diff-used and/ or nickel plated. Numerous othertypes of reed switches are available and reed switches which arenormally closed rather than open may also be obtained commercially.Because insulated connection is easily made to both ends of the reedswitch, it may be readily incorporated in positive or negative groundsystems.

Physical placement of the magnetically actuated reed switch 45 inrelation to the rotating magnet 50 and other elements of the system mayaffect the operation of the system. In FIG. 3, a particular way ofmounting the reed switch in a conventional distributor is shown. Thedistributor includes a housing 71 and a cap 72. A distributor shaft 73extends upwardly into the housing and drives a drive plate 75. Driveplate 75 is connected through the usual spring biased flyweights 77 anddrive pins 78 to a centrifugal advance plate 79.

Advance plate 79 drives the distributor bushing 92 and the rotor shaft81 upon which rotor 83 is mounted. Rotor 83 carries the moving contact15 of the distributor, and fixed contacts 14 of the distributor extenddownwardly in the interior of cap 72. Rotor shaft 81 carries a cam 85which may be the conventional type used to operate mechanical breakerpoints. A grommet 87 of rubber, plastic or other suitable material, isfitted over cam 85 to be driven thereby and carries an annular magnet89. Magnet 89 may be of powdered barium ferrite and is magnetized aboutthe periphery thereof in alternate evenly spaced poles. The magnet neednot be annular, however, and may be constructed without alternate poles.Instead of a rotating magnet, a fixed magnet with a rotating piece ofsoft iron might sufflce.

It has been found under certain circumstances that the reeds 47, 48 ofthe reed switch tend to bounce at their resonant frequency. Naturallythis can cause faulty operation of the ignition system. Such bouncingmay be the result of flux variations of the resonance frequency producedby vibrations. Such vibrations may be due to minute changes of gapbetween to periphery of magnet 50 and the reeds 47 and 48. v This canoccur as the result of play in the distributor bush-ing 92 within thetolerance requirements of the structure, for example, 2 to thousand-s ofan inch. The magnet 50 may have a peripheral eccentricity which varieswithin several thousands of an inch. Finally, vibration of thedistributor housing causing the distributor shaft and reed switchhousing to flex slightly may also set up vibrations to producevariations in flux. As the distance between the magnet and the reedchanges with vibration, the flux changes produced, or some harmonicfrequencies of these flux changes, approach the resonance frequency ofthe reed itself. It takes very little driving force to cause the reedsto vibrate or bounce at the resonance frequency.

This problern'is eliminated by utilizing a high frequency flux filter101. Filter 101 may be comprised of a copper bushing or sleeve or may bea multi-turn winding surrounding the reed switch 45. The space betweenthe glass capsule of reed switch 45 and the sleeve may be filled withepoxy 102 (see FIG. 2) to provide a secure mechanical mounting. Thesleeve is secured by brackets 104 and 105 to the vacuum advance plate103 of the distributor and may be advanced in a known manner. Thebushing or winding acts as a shorted turn to filter out flux variations.The inductance of the magnetic shield must be low such that it willaffect only the higher frequencies, those frequencies of the order ofthe nautral resonance frequency of the reed switch 45. The resonancefrequency of the reed switch may, for example, be of the order of 2200cycles per second while the maximum frequency of the firing pulses mayapproximate 330 cycles per second. The resistance and turns of theshielding must be such that the back m'agnetomotive force producedthereby should block the high frequency components of the main pulseflux. By utilizing such shielding, the likelihood of bouncing in thereeds of the reed switch is substantially reduced. With a reed switchresonance frequency of the order of 2200 cycles per second and a maximumoperating frequency of the reed switch of the order of 330 cycles persecond, a satisfactory sleeve thickness has been found to be of theorder of one eighth inch.

It may therefore be seen that the invention provides an improvedelectrical system utilizing a magnetically actuated reed switch. Thevibrations of the reed switch which may be cause-d by spurious fluxvariation are minimized for superior performance of the system. Theinvention has been shown in connection with an ignition system. However,any system wherein a reed switch is to be operated at a frequecysubstantially lower than the resonance frequency of the reed switch andwherein stray fields may cause spurious flux variation at the resonancefrequency of the reed switch, may effectively utilize the invention.

I claim:

1. A switching structure for use in an electrical system which may besubject to the influence of magnetic fields exhibiting variations influx, said switching structure including in combination, a reed switchcomprising a pair of magnetic reeds encapuslated within a glass tube andoperable to control a function of the electrical system, a copper sleevedirectly surrounding said glass tube, movable magnetic means operable toprovide a magnetic flux through said reeds varying in a given frequencyrange to open and close said reed switch, said reed switch having anatural resonance frequency which is substantially higher than the givenfrequency range, said copper sleeve forming a shorted turn about saidreeds and being of a thickness to provide a sufficiently low inductanceto have a negligible effect of magnetic flux variations within the givenfrequency range, and acting to filter out magnetic flux variations atfrequencies of the order of said natural resonance frequency.

2. An ignition system for an internal combustion engine, including incombination, a switching circuit and means responsive thereto to producefiring pulses for the engine, said switching circuit including a reedswitch ope-rable to control the frequency at which firing pulses areproduced, a permanent magnet annular disc having alternate north andsouth poleslocated about its periphery, said magnetic disc beingpositioned proximate said reed switch and having eccentricityvariations, a rotatable shaft fixed to said disc for rotating the samein synchronism with the engine for producing a varying magnetic fluxthrough said reed switch, to open and close said reed switch to producefiring pulses within a range of frequencies, said reed switch having anatural resonance frequency which is substantially higher than themaximum frequency of the firing pulses, said magnetic disc and saidshaft producing variations in fiux having a frequency of the order ofsaid natural resonance frequency due to vibration and the eccentricityof said disc and a magnetic shield surrounding said reed switch, saidmagnetic shield forming a shorted turn about said reed switch and havingan inductance sufficiently low to have a negligible effect on magneticflux variation below the maximum frequency of firing pulses andoperating to filter out magnetic flux variations at frequencies of theorder of said natural resonance frequency. V

3. The combination of claim 2 wherein the natural resonant frequency ofsaid reed switch is of the order of 2200 cycles per second, and saidmagnetic shield comprises a copper sleeve of the order of one eighth.inch thick.

which may be subject to the influence of magnetic fields exhibitingvariations in magnetic flux, said switch struc- 4. A switching structurefor use in an electrical system 5 ture including in combination, amagnetic reed switch operable to control a function of the electricalsystem, a magnetic shield surrounding said reed switch and forming atleast one shorted turn about said reedswitch, permanent magnet meansoutside said shield and movable to provide a magnetic flux through saidreed switch which varies at frequencies up to a given firequency tooperate said reed switch, said reed switch having a natural resonantfrequency which is substantially higher than the given frequency ofoperation resulting from said magnet means, said magnetic shield havingan inductance sufficiently 140W to have a negligible eifect on magneticflux variations at the operating frequencies and acting to filter outflux variations at frequencies of the order of said natural resonantfrequency.

References Cited by the Examiner UNITED STATES PATENTS 2,614,188 10/1952 Williams et al 2Q093 X 2,781,412 2/1957 Mike 2-00-19 X 2,938,9825/1960 Brown et al 20'087 X 3,005,069 10/1961 Sippach et a1 20'()138 X10 3,187,244 6/1965 Sum-merer 200--19 X BERNARD A. GILHEANY, PrimaryExaminer.

.T. J. BAKER, Assistant Examiner.

1. A SWITCHING STRUCTURE FOR USE IN AN ELECTRICAL SYSTEM WHICH MAY BESUBJECTED TO THE INFLUENCE OF MAGNETIC FIELDS EXHIBITING VARIATIONS INFLUX, SAID SWITCHING STRUCTURE INCLUDING IN COMBINATION, A REED SWITCHCOMPRISING A PAIR OF MAGNETIC REEDS ENCAPSULATED WITHIN A GLASS TUBE ANDOPERABLE TO CONTROL A FUNCTION OF THE ELECTRICAL SYSTEM, A COPPER SLEEVEDIRECTLY SURROUNDING SAID GLASS TUBE, MOVABLE MAGNETIC MEANS OPERABLE TOPROVIDE A MAGNETIC FLUX THROUGH SAID REEDS VARYING IN A GIVEN FREQUENCYRANGE TO OPEN AND CLOSE SAID REED SWITCH, SAID REED SWITCH HAVING ANATURAL RESONANCE FREQUENCY WHICH IS SUBSTANTIALLY HIGHER THAN THE GIVENFREQUENCY RANGE, SAID COPPER SLEEVE FORMING A SHORTED TURN ABOUT SAIDREEDS AND BEING OF A THICKNESS OF PROVIDE A SUFFICIENTLY LOW INDUCTANCETO HAVE A NEGLIGIBLE EFFECT OF MAGNETIC FLUX VARIATIONS WITHIN THE GIVENFREQUENCY RANGE, AND ACTING TO FILTER OUT MAGNETIC FLUX VARIATIONS ATFREQUENCIES OF THE ORDER OF SAID NATURAL RESONANCE FREQUENCY.